Process of preparing a zinc sulfide powder coated with tritiated polystyrene



United States Patent M 3,342,743 PROCESS OF PREPARING A ZIN'C SULFIDE POW- DER COATED WITH TRITIATED POLYSTYRENE Joseph Rosenberg, Wellesley, Mass., assignor to Laboratory for Electronics, Inc., Waltham, Mass., a corporation of Delaware No Drawing. Filed Mar. 19, 1965, Ser. No. 441,312 1 Claim. (Cl. 252301.1)

The present invention relates in general to self-luminous compounds and a method of preparing the same and more particularly to a self-luminous compound in which the energizing source is tritium.

The use of self-luminous compounds, particularly in the form of luminous paint is now well known. Such compounds are frequently used to coat the faces of watches, clocks, and compasses as well as other selfilluminated surfaces such as sign lettering and the like. A self-luminous compound is a material which itself contains the energizing source for producing the illumination. Thus, a compound of this nature requires a relatively high luminosity in order to be clearly visible in the dark without the application of any external power. Most selfluminous compounds employ a mixture of a phosphor material together with a radioactive isotope. The former exhibits the characteristic of fluorescing and emitting visible light upon absorption of appropriate radiation. The earliest self-luminous compound employed, as the radioactive component, the element radium. However, luminous compounds formed with radium had several disadvantages. One such such disadvantage lies in the highly toxic nature of radium. Thus, preparation of radium paint involves a severe biological hazard to the people preparing the luminous surface and even the luminous surfaces are, to a degree, dangerous. More recently, self-luminous compounds have been formed using as the radioactive component of the mixture, the isotope tritium, which may be incorporated into any one of a number of convenient organic forms. One such selfluminous compound is described in U.S. Patent 3,033,797. As described therein, the luminous compound includes a phosphor such as zinc sulfide which is dispersed in a vehicle, typically a plastic binder. The tritium is then incorporated into the plastic which serves as the dispersing medium. The problem with this self-luminous compound arises both from the point of view of attaining a sufficiently high level of luminosity and from the point of view of stability of the luminous compound over long periods of time. It is apparent that when the luminous compound is used to coat commercial articles such as watches and compasses or to form self-illuminated signs such as highway signs and the like, the surfaces are exposed to a great variety of atmospheric and temperature conditions. It is important that the level of luminosity not decay sharply under these various conditions.

It is, therefore, the primary object of the present invention to provide a self-luminous compound characterized by a high level of luminosity and exhibiting extremely stable characteristics over long periods of time.

Another object of this invention is to provide a selfluminous compound in which the radioactive element does not present a significant biological hazard and in which the self-illuminated surface exhibits stable luminosity characteristic over periods of time.

Broadly speaking, the self-luminous compound of the present invention is formed by coating phosphor grains with tritiated polystyrene. The polystyrene is tritiated by a process which involves the reduction of phenylacetylene by either carrier-free tritium or a mixture of tritium and protium. The resultant monomer is then polymerized, dis solved and coated on the phosphor grains. The volatile 3,342,743 Patented Sept. 19, 1967 solvent used to dissolve the polystyrene is then evaporated otf. Such a compound exhibits a very high level of luminosity and has been found to maintain this level of luminosity at a substantially stable value (within of the initial value) over periods as long as one year. The luminous powder formed in this fashion is particularly suitable for use in a paint. For such purposes, it is mixed with a solvent and plastic binder. The resultant mixture may then be painted onto a surface and upon evaporation of the volatile solvent, a suitable self-luminous coated surface is produced. It should be noted that the solvent selected must be one which does not dissolve polystyrene. One class of solvents generally suitable for this purpose is the aliphatic alcohols. The binder material must be a plastic exhibiting both suitable adherent and chemically stable properties and again must be a material which is soluble in a solvent which does not dissolve polystyrene. Suitable class of binder materials have been found to be derivatives of cellulose.

Self-luminous compounds in accordance with the principles of this invention have been prepared using the following specific process. 624 milligrams of phenylacetylene was placed into a flask containing milligrams of 30% palladium on charcoal as a catalyst. The flask was attached to a vacuum line and a mechanical shaker. After evacuation of the air from the flask, a glass bulb containing 7.8 millimoles of protium was opened to the flask and the flask was shaken for two hours. The protium bulb was then closed and a bulb containing 17.2 millimoles (1000 curies) of carrier-free tritium was opened to the interior of the reaction flask. The reaction flask was then mechanically shaken for another 48 hours. At the conclusion of this shaking, the protium containing bulb was opened to the interior of the reaction flask, the tritium bulb was closed and shaking was continued for an additional 72 hours. The reaction mixture was then vacuum distilled into a previously carefully weighed flask and the flask containing the reaction mixture was then weighed again. To the 380 milligrams of distillate, 5 milligrams of benzoyl peroxide was added. Under reflux conditions, the mixture was placed in a 90 C. oil bath for 60 hours. The solid polymer was dissolved in benzene, assayed, and found to contain 500 curries of activity. The solution was then mixed thoroughly with 800 grams of zinc sulfide powder and then evaporated with agitation. The coated powder was then removed from the flask as a self-luminous compound.

The luminosity of this powder was measured with an Aminco photometer and was found to be 2.5 microlamberts per millicurie and other compounds measured in this same manner exhibited levels of luminosity no higher than 1 microlambert per millicurie. This measurement was for an infinite thickness of the powder, that is for such a thickness that when more powder was added the luminosity did not increase or decrease. Measurements were continued on this mixture for a period in excess of one year, during which the luminosity was found to decrease at the rate of about 10% per year. All other tritiated luminescent materials measured, exhibited a decrease of 25% to 40% per year.

The powder formed by a process, such as that described above, consists of grains of the phosphor material with each individual grain coated with the tritiated polystyrene. In order to preserve efficiency of this mixture as a selfilluminating compound, it is important, as briefly noted above, that these coated grains of phosphor are not placed in a solvent for the polystyrene. If the polystyrene coated phosphor grains were placed in such a solvent, then the phosphor material becomes dispersed in a vehicle of tritiated polystyrene. Under these conditions, the powder fails to achieve maximum luminosity characteristics. When used with a solvent and binder for a painting process, the powder should then be used With materials that are compatible with this requirement. From this requirement it follows that polystyrene cannot be used as the dispersed medium.

The invention having been described as various modification and improvements Will now occur to those skilled in the art and the invention herein should be construed as limited only by the spirit and scope of the appended claim.

What is claimed is:

The process of producing a self-luminous composition comprising the steps of: reducing phenylacetylene with a mixture of tritium and protium; polymerizing the resultant tritiated styrene monomer; dissolving said tritiated styrene in benzene; mixing said benzene polystyrene solution with zinc sulfide powder; evaporating said benzene with agitation to produce zinc sulfide powder in which the individual grains of powder are coated with tritiated polystyrene.

References Cited CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGE'IT, Examiner.

S. I. LECHERT, ]R., Assistant Examiner. 

